Re: [PATCH v3 3/3] mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim

[Yu Zhao <yuzhao@google.com>]

On Fri, Mar 31, 2023 at 1:08 AM Yosry Ahmed <yosryahmed@google.com> wrote:

...

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a3e38851b34ac..bf9d8e175e92a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -533,7 +533,35 @@ EXPORT_SYMBOL(mm_account_reclaimed_pages);
>  static void flush_reclaim_state(struct scan_control *sc,
>                                 struct reclaim_state *rs)
>  {
> -       if (rs) {
> +       /*
> +        * Currently, reclaim_state->reclaimed includes three types of pages
> +        * freed outside of vmscan:
> +        * (1) Slab pages.
> +        * (2) Clean file pages from pruned inodes.
> +        * (3) XFS freed buffer pages.
> +        *
> +        * For all of these cases, we have no way of finding out whether these
> +        * pages were related to the memcg under reclaim. For example, a freed
> +        * slab page could have had only a single object charged to the memcg
> +        * under reclaim. Also, populated inodes are not on shrinker LRUs
> +        * anymore except on highmem systems.
> +        *
> +        * Instead of over-reporting the reclaimed pages in a memcg reclaim,
> +        * only count such pages in system-wide reclaim. This prevents
> +        * unnecessary retries during memcg charging and false positive from
> +        * proactive reclaim (memory.reclaim).

What happens when writing to the root memory.reclaim?

> +        *
> +        * For uncommon cases were the freed pages were actually significantly
> +        * charged to the memcg under reclaim, and we end up under-reporting, it
> +        * should be fine. The freed pages will be uncharged anyway, even if
> +        * they are not reported properly, and we will be able to make forward
> +        * progress in charging (which is usually in a retry loop).
> +        *
> +        * We can go one step further, and report the uncharged objcg pages in
> +        * memcg reclaim, to make reporting more accurate and reduce
> +        * under-reporting, but it's probably not worth the complexity for now.
> +        */
> +       if (rs && !cgroup_reclaim(sc)) {

To answer the question above, global_reclaim() would be preferred.